This invention relates to improvements in a control apparatus for an elevator which is driven by an induction motor.
In a control apparatus for an elevator of the system wherein a commercial alternating current is converted into a direct current value by a converter, whereupon the direct current is inverted by an inverter into an alternating current of variable voltage and variable frequency with which an induction motor for hoisting a cage is driven, a method according to which regenerative power generated by the hoisting induction motor for the elevator cage is consumed within the induction motor has been proposed in Japanese patent application Laid-open Nos. 59-17879.
According to this document, in the powering mode of the elevator, the torque of the motor is controlled by the so-called "slip frequency control", while in the regenerative mode, control is performed so as to consume the regenerative power within the induction motor. The powering and the regeneration are changed-over when a slip frequency command signal has become zero.
FIGS. 4(a)-4(d) are characteristics diagrams showing the relationships between the operating speed v of the elevator and the slip frequency command signal f.sub.3. In FIG. 4(a) illustrative of the operating speed v of the elevator, the cage is started at a time t=0 and is accelerated to reach its full speed at t=t.sub.1. Subsequently, when the cage has come to a deceleration initiation point at t=t.sub.2, it begins to be decelerated, and it arrives at a destination floor at t=t.sub.3. As is well known, a counterweight corresponding to about 50% of a rated movable load is usually used in an elevator. Therefore, in a case where the cage is upwardly run carrying a load near the rated movable load, the slip frequency command signal f.sub.s continues to be zero during the deceleration as shown in FIG. 4(b). (Note: Strictly speaking, the slip frequency command signal f.sub.s is determined by the movable load, the acceleration and the moment of inertia.) In addition, in a case where the cage is run under a load equal to about half of the rated movable load, the slip frequency command signal f.sub.s continues to be zero during the full-speed running as shown in FIG. 4(c). Besides, the slip frequency command signal f.sub.s can continue to be zero during the acceleration in a case where the cage is downwardly run near the rated load, during the acceleration in a case where it is upwardly run near the no-load state thereof, or during the deceleration in a case where it is downwardly run near the no-load state thereof.
In general, the slip frequency command signal f.sub.s is instantly changed-over from plus to minus or vice versa as shown in FIG. 4(d). As stated above, however, the periods during which the slip frequency command signal continues to be zero exist under the specified conditions.
With the prior-art control apparatus for the elevator, the change-over between the powering and the regeneration is executed when the slip frequency command signal is zero. Therefore, when the slip frequency command signal continues to be zero, the change-over between the powering and the regeneration arises frequently, and the torque of the motor becomes unstable. As a result, the comfort of riders in the elevator is reduced.